The present invention generally relates to optical
for detecting changes in a magnetic field. More specifically, the invention relates to apparatus and methods for detecting movement of a target and/or for detecting variations in a magnetic field such as may result from movement of a target. For example, the invention is particularly useful for determining the speed and/or torque of a rotating member such as a turbine engine shaft.
The aerospace industry is an excellent example of an advanced technology industry wherein the use of optics for sensors and controls is destined to become the preferred technology on the next-generation aircraft. Optical sensors and controls utilize various properties of electromagnetic radiation to monitor events such as, for example, movement of flight controls, rotational speed and torque of a shaft, and for use in proximity detectors. Such apparatus and methods that utilize optics enjoy significant advantages over conventional electrical and magnetic sensors. Most important is that an optics based system has a significantly greater immunity to electromagnetic interference. Optical sensors also tend to be very compact and lightweight.
Optical sensors typically rely on modulating or changing a characteristic of electromagnetic energy and use a s photoelectric device to convert that modulated characteristic into an electrical signal that can be further processed with conventional electronics. For example, optical speed sensors are known that utilize the Faraday rotation effect to detect the rotational speed of an object. Such an apparatus is shown in U.S. Pat. No. 4,947,035 issued to Zook et al. The Zook et al. device uses linearly polarized light that is transmitted through a bulk magneto-optic material that exhibits the Faraday effect in the presence of a magnetic field. In particular, the Zook et al. device detects the speed of a rotating body by modulating a magnetic field established by a permanent magnet using highly permeable teeth s that rotate with the body and are in close proximity to the magnet. The teeth cause a modulation in the magnetic field flux lines that are parallel to the propagation axis of the polarized light. As a consequence, the polarized light undergoes a Faraday rotation of its polarization angle in proportion to the changing magnetic field. An output polarizer is then used to detect the polarization angle change relative to the input polarization angle. A similar arrangement for providing an optical switch is shown in U.S. Pat. No. 4,843,232 issued to Emo et al. Other examples of s optic sensors are disclosed in U.S. patent applications Ser. Nos. 07/709,352 and 07/709,627 owned by the assignee of the instant application.
Although a speed sensor such as shown by Zook et al. exhibits many of the aforementioned advantages of optical o sensors, a significant cost driver associated with such devices is the need for polarized light. In particular, polarizers are expensive materials and are not immune to harsh environments such as high temperatures within a turbine engine, for example. It is, therefore, an object of the present invention, to provide an optical speed and torque sensor that does not require polarizers or the use of polarized electromagnetic energy. In a broader sense, it is an object of the present invention to provide an apparatus and method for detecting changes in a magnetic field using an optical sensor that does not require polarization of electromagnetic energy used in the sensor or the use of an output polarizer.